Magnetic field directional sensors, such as Giant Magneto Resistive (GMR) or Anisotropic Magneto Resistive (AMR) sensors, for example, are used in a wide variety of applications. FIGS. 1A-1B show a how a magnetic field directional sensor 100 (e.g., GMR sensor) behaves under different magnetic fields. The magnetic field directional sensor 100 includes a nonmagnetic conducting middle layer 102 (e.g., an ultrathin copper layer) sandwiched between first and second ferromagnetic alloy layers 104, 106. An artificial anti-ferromagnetic layer 107 is disposed under the second ferromagnetic allow layer 106. Additional layers can also be included. As will be appreciated in more detail below, the resistance of the magnetic field directional sensor 100 varies depending on the magnitude and direction of a magnetic field applied to the sensor 100.
FIG. 1A shows a condition where no external magnetic field is applied to the sensor 100. Under this condition, the magnetic moments of the first and second alloy layers 104, 106 face opposite directions (see arrows 108, 110) due to anti-ferromagnetic coupling, and the current 112 attempting to pass through the sensor 100 encounters a large resistance.
In contrast, in FIG. 1B an external magnetic field as shown by arrow 114 has been applied to overcome anti-ferromagnetic coupling. Within the first and second alloy layers 104, 106; this magnetic field 114 tends to align the magnetic moments, as shown by arrows 116, 118. As a consequence, the current 120 attempting to pass though the sensor 100 in FIG. 1B encounters a low resistance, relative to FIG. 1A. Thus, by monitoring the resistance of the sensor 100, the magnitude and/or direction of a magnetic field (e.g., 114) can be evaluated.
One particular application of interest for GMR/AMR sensors is determining an angular position of a rotating shaft. In conventional solutions, a permanent magnet, sometimes referred to as a “button” magnet, can be mounted to an end of a rotating shaft so as to be centered on the shaft's axis of rotation. FIGS. 2A-2C show top views of such a button magnet 202 as it rotates, wherein a magnetic field directional sensor 200 (e.g., GMR or AMR) is positioned there over so as to remain stationary as the button magnet 202 rotates there under. Within the confines of the button magnet 202, magnetic field lines 204 are straight and parallel to one another. As the shaft (and hence button magnet 202) rotate, the directionality of the magnetic field lines 204 passing though the sensor 200 change accordingly. For example, in FIG. 2A (e.g., a 0° angular position) the magnetic field lines 204 tend to align the magnetic moments of layers in the sensor 200, thereby creating a low resistance state in the sensor 200. However, as the magnetic field lines 204 rotate (FIG. 2B—a 90° angular position, FIG. 2C—a 180° angular position) they no longer align the magnetic moments as strongly, and the resistance of the sensor 200 can increase proportionately. Because the resistance of the sensor 200 changes depending on the directionality of the magnetic field lines 204 passing there though, a controller (not shown) can measure the resistance of the sensor 200 over time and continuously correlate the resistance at any given time to a corresponding magnetic field direction. In this way, the controller can determine the rotational angle of the shaft as the shaft rotates. In other cases the angle calculation is completed by an intelligent state machine and firmware included in the sensor's manufacturing process. How the angle is ultimately calculated is dependent on the level of sensor integration.
Unfortunately, in many applications, it isn't feasible to mount a circular button magnet on an end of a rotating shaft, for example, due to space concerns or due to some other component needing to be mounted to the end of the shaft. For example, in vehicles, both ends of an electric drive motor shaft may be used to drive two independent systems leaving no access at the shaft ends for a magnet and sensor.
Therefore, improved angle sensing techniques are needed wherein angle sensors are disposed along the length of a shaft, rather than at an end of the shaft.